It has been a long standing objective in the fermentation arts to isolate and identify or to otherwise secure species and strains of yeast that are particularly adapted and well suited to the fermentation processes used in the production of alcohols. Yeasts particuarly suited to such applications must ferment the substrate medium in a reasonable period of time to produce between four and twelve per cent weight by volume of ethanol. The yeast should be readily cropped at the end of the fermentation and the collected cell mass should be sufficiently viable that it can be utilized to pitch subsequent fermentations with a high degree of confidence. Perhaps most important to a commercial scale operation where the repeated and constant quality of the yeast's performance is paramount, however, is the genetic stability of the yeast strain. Such strains must lend themselves to repeated use over long periods of time without incurring any substantial genetic changes. One attempt at producing an amalgam of the strong attenuating characteristics of a wild type, non-brewing, yeast strain with the otherwise desirable fermentation characteristics of an established brewery yeast is described in U.K. Patent Specification 1,212,437. In that patent, Windisch et al describe the production of Z16 diploid hybrids which are produced from classical hybridization techniques in a hybrid cross between a haploid B12/14 strain of Saccharomyces uvarum (carlsbergensis) brewers' yeast and a wild type Z1-2C strain having the ability to attenuate dextrins. While the recrossing of the resulting Z16 hybrids allows a good deal of versatility in addressing the various problems encountered in differing fermentation systems, the hybrid strain only has a diploid genome. In this latter respect, the Z16 hybrids do not lend themselves well to commercial scale applications.
It will be appreciated that a/.alpha. mating type diploids are in effect "neuter" strains with little or no propensity towards sexual activity and, from that point of view, may be considered to be genetically stable. Such mating type diploids, however, are extremely susceptible to environmental stress such as, for example, starvation, and respond to these commonly encountered stresses by producing mixtures of viable a and .alpha. mating type haploid spores.
Thus, not only are the carefully engineered advantages of the original diploid hybrid lost, but under more favourable environmental conditions, those spores mate to produce arbitrary recombinations of diploid genotypes. This is particularly detrimental when such mating involves contaminant organisms whose dominant gene characteristics are then incorporated into the resulting diploids, in view of the completely unpredictable results that ensue from such a cross.
a/a and .alpha./.alpha. mating type diploid hybrids are even less stable than the a/.alpha. types in that the strains which are homozygoris for the mating type allele are additionally prone to diploid sexual recombination.
The Z16 diploid genome therefore is subject to statistical variation that on the balance of probabilities is likely to lead to fundamental changes in both the character and quality of the product produced in the fermentation process over a period of time. In this latter regard, it will be remembered that industrial yeast strains, including brewing and distilling strains, are often polyploid or even aneuploid and, as a consequence, do not possess mating types, have a low degree of sporulation and have a low degree of spore viability, all of which contribute to their overall genetic stability. The widespread use of such polyploid yeasts for industrial purposes, therefore, does not appear to be merely an historical artifact. Furthermore, owing to their multiple gene structure, polyploids are genetically more stable and less susceptible to mutational forces than either haploid or diploid strains. This allows such polyploid yeasts to be used routinely with a much higher degree of confidence in the expectation of their continued and consistent performance than is possible with haploid or diploid strains.
More recently an attempt has been made, see R. S. Tubb et al, EBC Congress 1982, pg. 487, at crossing a poly-diploid brewers' yeast strain of Sacch. uvarum (carlsbergensis), NCYC 1324, with a and .alpha. mating type haploid strains of Sacch. diastaticus, using rare mating techniques coupled with respiratory markers to indicate successful hybridizations.
One of these latter crosses utilized a Sacch. diastaticus haploid strain, BRG 160B which is characterized as being a producer of extracellular glucoamylase. The resulting hybrid, however, showed only a marginally improved fermentative potential over the Sacch. uvarum (carlsbergensis) strain.
In another cross, yet another Sacch. diastaticus haploid strain was used, BRG 514A which is a known producer of extracellular glucoamalyse. The resulting hybrid was also found to produce ferulic acid decarboxylase, an enzyme which decarboxylates ferulate to produce 4-vinylguaiacol which in turn contributes a phenolic off-flavour to beverages containing that substance. This latter hybrid was then induced to sporulate in the hope that reducing the ploidy of the hybrid would eliminate certain characteristics which are considered undesirable in the brewing of potable alcohols. Once the asci were dissected and the few viable spores isolated and germinated, one strain was found to be devoid of these undesirable characteristics and yet continued to be capable of producing extracellular glucoamylase. None of the hybrids produced by Tubb et al, however, were capable of degrading any more than about 25% of the wort dextrins, since the dextrinase enzymes produced by these hybrids were incapable of hydrolysing .alpha.-1,6 glucosidic linkages.
Attempts have also been made at crossing a polyploid Sacch. uvarum (carlsbergensis) brewing strain with a polyploid Sacch. diastaticus strain using somatic fusion techniques. Somatic fusion techniques are generally disclosed in U.S. Pat. No. 4,172,764 - Heslot et al. While fusion products could be formed, fermentative ability was poorer than that of the Sacch. uvarum (carlsbergensis) parent and both maltose and dextrin fermentation rates were reduced in comparison with those of either of the fusion partners.
It is an object of the present invention to provide a novel allopolyploid yeast strain suitable for the production of fuel alcohols.